High quality lactam is used as raw material for polyamide fibers and polyamide resins. It is well known that the lactam is produced in the following manner. That is, oxime is rearranged in a sulfuric acid medium, then neutralized with aqueous ammonia and extracted with an aromatic hydrocarbon solvent such as benzene, followed by removing the solvent by distillation and then subjecting the residue to rectification ("Kogyo Yuki Kagaku (Industrial Organic Chemistry)", page 244 published from Tokyo Kagaku Dojin in 1989).
However, in the case of oxime being rearranged in gas phase using a solid acid in place of sulfuric acid as a catalyst, when the resulting crude lactam is dissolved in water and the solution is mixed with an aromatic solvent such as benzene, lactam cannot be extracted into the aromatic solvent layer probably because the aqueous solution does not contain inorganic materials and the above process which is effective in the case of oxime being rearranged in a sulfuric acid medium cannot be applied to the production of lactam when a solid acid catalyst is used.
When oxime is rearranged in gas phase using a solid acid catalyst and when the resulting crude lactam is subjected to rectification, lactam of satisfactory quality cannot be obtained probably because by-products differ from those in the case of oxime being rearranged in the sulfuric acid medium.
Another effective process for producing high quality lactam comprises treating a crude lactam obtained by Beckmann rearrangement in fuming sulfuric acid with hydrogen at 50.degree.-80.degree. C. using a palladium or nickel supported catalyst. According to this process, when the hydrogen treatment temperature is 130.degree. C., not only sufficient purification effect cannot be obtained, but also quality of lactam is degraded (U.S. Pat. No. 5,032,284).
However, even when crude lactam obtained by gas phase Beckmann rearrangement is subjected to hydrogen treatment at 50.degree.-80.degree. C., no lactam of satisfactory quality can be obtained probably because the by-products differ from those in the case of employing liquid phase rearrangement.